If the 20th century was the story of a swelling human tide, the 21st is the story of the ebb. Birth rates are sliding in most of the world, populations are ageing fast across advanced economies and China, and the economic model built on ever-more workers and ever-cheaper capital is creaking.

This isn’t doom-mongering; it’s the mainstream forecast. The UN’s latest revisions show the global fertility rate has already fallen to roughly 2.25 and is projected to sink toward or below replacement (≈2.1) by mid-century, then lower still by 2100. The practical upshot: fewer countries with growing working-age populations and many more coping with shrinkage and old-age dependency.

The Lancet’s 2024 global study sharpened the point: by 2050, about three-quarters of countries are expected to be below replacement fertility; by 2100, almost all will be. Births will concentrate increasingly in today’s lower-income nations, especially in sub-Saharan Africa, while most of the world faces structural decline.

Why is this happening?

1) Later marriages, fewer children, higher costs. Urban living, dual-career households, housing/childcare costs, and changing preferences all push fertility down.

2) Education and empowerment. Female education and labour-force participation (unequivocally good in themselves) correlate with smaller family sizes.

3) Health and environment. A separate, but related, concern is male reproductive health. The best current evidence is that sperm counts have declined globally, with the sharpest documentation historically in North America/Europe/Australia, and more recent meta-analysis showing declines also in South/Central America, Asia, and Africa. Put plainly: the problem is not just “in the West”.

4) Policy arithmetic. In many advanced economies, tax/benefit systems, housing markets and childcare provision unintentionally penalise family formation.

“Peak human” meets economics

Demography is not destiny, but it’s a powerful headwind or tailwind.

Labour and growth. The OECD and IMF both warn that ageing populations, absent policy change, will slow GDP-per-capita growth, tighten labour markets, and strain public finances as the worker-to-retiree ratio falls. Migration, longer working lives and higher participation (especially among women) can mitigate, but not fully erase, the drag.

Asset prices (yes, this matters for your pension and house). The “asset-meltdown” thesis is sometimes overstated, but a large body of research finds ageing and shrinking cohorts create headwinds for asset returns, especially housing in greying societies, because fewer prime-age buyers are competing for the same stock while older cohorts gradually decumulate. BIS work estimates demographic headwinds on US real house prices of ~80 bps per year, with even stronger effects in Europe and Japan. Reviews since then generally confirm significant (if heterogeneous) demographic effects on housing and broader asset markets. Translation: slower demand growth tends to mean lower trend returns and softer price support over time.

Public finances. Fewer workers + more retirees = rising pension and healthcare burdens unless productivity and participation jump, or immigration and reform fill the gap. Think higher effective tax rates, later retirement ages, or leaner promises.

West vs. South/East: diverging population paths

Advanced economies & China: Fertility well below replacement; median age rising rapidly; labour pools set to contract without migration/automation gains.

South & East (esp. sub-Saharan Africa and parts of South Asia): Still relatively high fertility (though falling), youthful populations, and the potential for a demographic dividend, if education, jobs, and governance keep pace.

On male fertility, earlier studies focused on Western declines; updated analyses now detect declines across regions, including the global South and East. There isn’t robust evidence of rising sperm counts in those regions; if anything, the trend looks downward worldwide, with environmental factors (e.g., endocrine-disrupting chemicals) plausible contributors.

The politics: demographic decline and anti-immigration rhetoric

Here’s the awkward linkage. Many Western economies are ageing and shrinking, yet large parts of the political conversation push for lower immigration just as business and public services face chronic labour shortages. The OECD and IMF are blunt: migration already helps sustain working-age populations, eases shortages, and raises output; turning off that tap without offsetting fertility or productivity miracles worsens the demographic squeeze. Smart immigration and integration policies, not blanket restrictions, are what the arithmetic demands.

What we can do (and what the future likely looks like)

1) Make family formation easier. Housing supply, affordable childcare, parental-leave design, and tax/benefit reform can all raise the “revealed” number of children families actually want. (Pronatal subsidies alone rarely move the needle much without these complements.)

2) Invest in human capital and productivity. With fewer workers, output per worker must carry more of the load, education, skills, R&D, and diffusion of best tech/practices.

3) Embrace longer, healthier working lives. Align retirement ages with longevity and support mid-career retraining and flexible work for older workers.

4) Calibrate migration to needs. Target shortages; recognise migrants’ entrepreneurial contributions; invest in integration, language, credentialing, housing.

5) Expect different markets. With ageing, base-case assumptions for asset returns and house-price appreciation should be lower than the late-20th-century norm, especially in Europe/Japan-style ageing societies. Global capital will likely chase youthful markets where institutions are strong enough to harness their demographic dividend.

The bottom line (and a personal note)

Demography won’t collapse civilisation, but it will reshape economies, budgets and portfolios. In greying societies, we should expect slower growth, tighter fiscal math, lower trend asset returns, and more reliance on productivity gains and migration to maintain living standards.

And yes, have kids if you want them (and if policy makes it feasible). As a matter of maths, two children roughly replace two parents; one child implies the population falls by one in the next generation. Elon Musk bangs this drum loudly; beneath the bombast sits a real arithmetic. But children are not policy instruments, they’re people. Societies that make it easier (not harder) to build a family will weather “peak human” more gracefully.

If we can align family policy, productivity, and smart migration, and drop the reflexive, self-defeating anti-immigration posture where labour markets are crying out, we’ll manage the ebb every bit as well as we surfed the tide.